The present invention relates to a color cathode-ray tube having a shadow mask and, more particularly, to the structure of the shadow mask of the color cathode-ray tube.
In a conventional color cathode-ray tube, a funnel section having a neck extending along the tube axis is hermetically sealed to a glass panel section, in such a way as to form an envelope. In-line electron guns are provided for emitting electron beams of the primary colors of red, blue and green; and a convergence device for super-imposing the electron beams from the electron guns are received in the neck. A deflection yoke device for deflecting electron beams from the electron guns is arranged around the funnel section. A phosphor screen is formed on the faceplate of the glass panel section and emits light upon the landing of electron beams on red, blue and green phosphor dots. A shadow mask is arranged within the glass panel section, in such a way as to oppose the phosphor screen. The plate portion of the shadow mask has a number of apertures which are regularly arrayed on the horizontal and vertical axes, respectively. The skirt portion of the shadow mask extends from the periphery of the plate portion along the tube axis.
The shadow mask is an important member which has a color selection function in determining the landing positions of the electron beams on the phosphor screen. In other words, when the electron beams converge at the correct angle, the shadow mask allows for the passage of the incident electrons, in such a way that each electron beam lands on its respective color dot on the phosphor screen, without straddling the other two color dots. When the electrons are incident on the shadow mask at an incorrect angle, the shadow mask interrupts the electrons.
When the array direction of a plurality of electron guns arranged in an in-line arrangement and normal to the tube axis is defined as a horizontal axis and the direction normal to the horizontal axis and the tube axis is defined as a vertical axis, the ratio of the size of the plate portion along the horizontal axis to that along the vertical axis is set at about 4:3. The plate portion has a curved surface corresponding to the phosphor screen and the curved surface is defined as a spherical portion of either a constant radius of curvature of a compound radius of curvature involving different radii of curvature. As shown in FIG. 1; in the plate portion of the shadow mask, when a radius of curvature along the horizontal axis and that of along the vertical axis are designated by RH and RV, respectively, the following relation exists: EQU RH&lt;RV (1)
This is shown in, for example, in U.S. Pat. No. Re. 27,259 (Japanese Utility Publication No. 50-6292), issued on Nov. 28, 1971, which discloses a design method of the shadow mask. According to this design method, when the distance from a plane contiguous with the center of the shadow mask and perpendicular to the tube axis to a given point on the shadow mask is designated by z, and distances from the tube axis to given points on the shadow mask, along the horizontal and vertical axes, are designated by x and y, respectively, distance z is given by equation (2) or (3), as follows: ##EQU1## In this embodiment, the radius of curvature RH of the shadow mask along the horizontal axis is determined to be normally smaller than the radius of curvature RV thereof along the vertical axis, and may be given, for example, by the following equation (4): EQU RH=0.88RV.+-.10% (4)
As given by equation (1), the radius of curvature RV of the shadow mask along the vertical axis is determined to be greater than the radius of curvature RH along the horizontal axis, for the following reason.
When the shadow mask is designed by a central radius of curvature RO which is equal to an axial distance between the deflection center and the center of the plate portion, one may assume that the center radius of curvature RO provides a complete sphere, and the distance between the deflection center and any region on the plate portion is constant. In this case, the electron beams for red, blue and green are converged in the aperture of the shadow mask, to provide a screen with low distortion in the electron beam spot. In this case, the phosphor screen should be designed to be a complete sphere having a constant radius of curvature. However, when the phosphor screen is formed into the complete sphere portion, it is difficult to comfortably monitor a picture image. In practice, therefore, the radius of curvature RV or RH of the plate portion is set to be greater than the axial distance RO between the center of the plate portion and the deflection center. That is, the plate portion is therefore kept as flat as possible. However, a convergence error is produced in the peripheral region of the phosphor screen due to the difference between the preset radius of curvature RV or RH and the axial distance RO between the center of the plate portion and the deflection center. In the vertical direction, a lateral distance from the center of the plate portion to the periphery thereof is relatively small, and the convergence error is relatively small. Accordingly, the radius of curvature RV in the vertical direction may be so set as to be relatively great and the plate portion may be formed into a relatively flat surface in the vertical direction. On the other hand; in the horizontal direction, since a longitudinal distance from the center of the plate portion to the periphery thereof is relatively great, the radius of curvature RH in the horizontal direction must be so set as to be smaller than that in the vertical direction RV, due to the correction limit of a dynamic convergence device which corrects the convergence error.
However, in the color cathode-ray tube described above, the distance between the plate portion of the shadow mask and the phosphor screen (which distance must be kept at a predetermined value, which is referred to as the Q value) is changed, due to the thermal deformation of the shadow mask during the operation of the color cathode-ray tube. If such thermal deformation exceeds the beam landing tolerance, one electron beam lands over two phosphor dots, causing color misregistration. In a general color cathode-ray tube having a shadow mask, the number of electrons passing through the aperture is about 1/4 of the total number of the electrons emitted from the electron gun, the remainaing electrons being interrupted by the shadow mask and heating the shadow mask. Meanwhile, the shadow mask comprises a metal thin plate having a thickness of from 0.1 to 0.3 mm and made of iron due to ease in forming the apertures and to manufacturing cost. For these reasons, the plate portion which is thus heated exhibits the doming phenomenon due to thermal expansion wherein it is deformed to project outward, as indicated by the imaginary line in FIG. 2. This results in a change in the Q value and degrades the color purity.